I think that you are right that NC is often a process not an event.
You are effectively NC and it seems your DCs are as well.
Well done for getting this far.
Who has the once a month visit to your house (minus DCs) been “agreed” with? I doubt the PIL have agreed to this. And I wonder what your DH is expecting - that if they behave for these visits he will then start nagging you to leave the DCs each month? And then it will be each week, everyday….and during that time they will continue to alienate, insult, sabotage and subjugate YOU as their mother by their subtle eroding comments and behaviours.
It’s really important that you keep educating yourself about dysfunctional and abusive families.
These people (intact the whole extended family system) are a threat to your own precious little family, your marriage and your position as a mother.
They will undermine your MH and drain your finite emotional energy and headspace which you need every ounce to raise your DCs.
You are miles ahead of your DH. You are much more emotionally informed, intelligent and resilient than your DH and your DCs need you to keep leading on this.
Look at his family as toxic - sick - with a contagion - they are all infected and keep reinfecting each - you can see this and are moving your DCs out of harms way.
They will never change. They are wired that way - so they will always behave the way that they do - they don’t understand normal relationships and will only become enraged the more you discuss it…..and so will you as you become more and more frustrated.
See them as a dangerous pack of animals to keep your marriage, family, children and MH away from.
Minimise exposure to them.
Maybe see the siblings as “uneducated” - and subconsciously working to preserve their own comfort. Have pity for them - but just close them down - time and time again.
So forget words with any of the ILs. Forget your DH being at the same place as you right now - you need to keep taking actions that are non negotiable. “No - that doesn’t work for me” to your DH, “Not going to discuss that” (and then change subject) with siblings.
Be resolute in your vision and keep taking actions to ensure that your DCs have a calm and peaceful home that is not dominated or infected by these people.
Need firm, calm, assertive, non emotional boundaries, actions and consequences for your DH.
Just keep taking actions - rather than doubting yourself - less talking etc - you will feel much more in control and less anxious.
Get a playbook ready so it is clear what is done in each situation. Then you can’t be blindsided. Draft this up with the help of rational people not your DH. You need to model good sound calm boundaries to your DH - and also that the world won’t fall in if you dare to state a preference of how you want to run your day.
It is likely to be tough in the early days - but being consistent with consequences and resolute in your own convictions - just weather the inevitable storm - let them kick up dust - it will settle and in time you will all be much happier.
If DH continues to see the ILs then IMHO there needs to be boundaries around who what where and when - something like - no DCs, not at your house, x times a month, no information to them about your lives and you don’t want any info coming back to pollute your relationship and trigger you. Also no “crisis” interruptions - everything is “will get back to you” - and then pass on to another sibling to deal with.
He needs to learn to manage them - because you have taken the brunt of it to date.